1. Field of the Invention
The present invention relates to the semiconductor manufacturing process, and more particularly, to a method of forming an intermetal dielectric (IMD) layer which can block the diffusion of mobile hydrogen ions.
2. Description of the Related Art
Nonvolatile memory cell arrays such as EPROMs, FLASH EPROMs and EEPROMs have gained widespread acceptance in the industry. It has been observed that there are data retention problems in nonvolatile memory cell arrays. It has been postulated that the poor data retention is due to mobile hydrogen ions that diffuse to the floating gate in the nonvolatile memory cell and cause the charge on the floating gate to be lost. For example, a passivation layer (of a high dielectric reflowable material such as phosphosilicate glass or borophosphosilicate glass) is formed on the wafer. The manufacturing process for forming the passivation layer causes the mobile hydrogen ions to be introduced. The mobile hydrogen ions can penetrate intermetal dielectric (IMD) layers to approach the floating gate in the nonvolatile memory cell, seriously affecting device reliability. Thus, providing an intermetal dielectric (IMD) layer that can block the diffusion of the mobile hydrogen ions is called for.
FIGS. 1A˜1B illustrate the formation of an intermetal dielectric (IMD) layer of the prior art. In FIG. 1A, a substrate 10 including a memory cell such as a FLASH cell is provided. In order to simplify the illustration, the conventional memory cell is not shown. A plurality of metal wires 12 are formed on the substrate 10. Then, using high-density plasma chemical vapor deposition (HDPCVD), a first SiO2 layer 16 is formed on the metal wires 12 and the substrate 10.
In FIG. 1B, using plasma enhanced chemical vapor deposition (PECVD), a second SiO2 layer 18 is formed on the first SiO2 layer 16. Then, a planarization is performed to smooth the surface of the second SiO2 layer 18. Thus, a conventional intermetal dielectric (IMD) layer 20 including the first SiO2 layer 16 and the second SiO2 layer 18 is formed on the metal wires 12 and the substrate 10.
Since silicon oxide layers cannot effectively block the mobile hydrogen ions that diffuse to the substrate 10, the traditional intermetal dielectric (IMD) layer 20 cannot solve the problem mentioned previously.